Method of checking the hermeticity of a closed cavity of a micrometric component and micrometric component for the implementation of the same

ABSTRACT

In order to check the hermeticity of a closed cavity of at least one micrometric component, said component includes a structure made over or in one portion of a substrate, a cap fixed to one zone of the substrate to protect the structure, and an indicator element whose optical or electrical properties change in the presence of a reactive fluid. The indicator element may be a copper layer for an optical check or a palladium resistor for an electrical check. The micrometric component is placed in a container which is then hermetically closed. This container is filled with a reactive fluid under pressure, which is oxygen for the optical check and hydrogen for the electrical check. The component in the container is subjected to a reactive fluid pressure higher than 10 bars for a determined time period, and to thermal (T&gt;100° C.) or optical (λ&lt;500 nm) activation. After this time period, an optical or electrical check of the indicator element determines the hermeticity of said cavity.

This application is a divisional of U.S. patent application Ser. No.10/595,946, filed May 22, 2006, which is a National Phase Application inthe United States of International Patent Application No.PCT/EP2004/012626 filed Nov. 8, 2004, which claims priority on EuropeanPatent Application No. 03026780.1, filed Nov. 21, 2003 The entiredisclosures of the above patent applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention concerns a method of checking the hermeticity of a closedcavity of at least one micrometric component. The micrometric componentincludes a structure made on or in a portion of a substrate and a capfixed onto one zone of the substrate for protecting the structure. Thecavity of the micrometric component is delimited by the inner surface ofthe cap, the structure and the substrate zone. This cavity can be filledfor example with an inert gas at a pressure close to atmosphericpressure or can be a vacuum cavity.

The invention also concerns the micrometric component with a closedcavity for implementing the checking method. The structure made in or ona portion of the substrate can be an integrated circuit or athree-dimensional structure or a combination of an integrated circuitand a three-dimensional structure.

BACKGROUND OF THE INVENTION

“Three-dimensional structures” of these micrometric components meanmicro-optoelectromechanical devices (MOEMS) or microelectromechanical(MEMS) devices, such as reed contactors, accelerometers, micro-motors,quartz resonators, sensors of micrometric size that have to be left tomove freely after encapsulation in a controlled atmosphere. Theconstruction of these three-dimensional structures of the micrometriccomponents can occur on an insulating substrate or on a semi-conductorsubstrate in which integrated circuits have, for example, been madebeforehand. In this latter case, it is possible to take the metalliccontact pads of the integrated circuits to start the deposition of themetallic layers that will, in part, form the structure of themicrometric component and enable it to be electrically connected to saidcircuit.

EP Patent No. 0 874 379 by the same Applicant discloses amicro-contactor with micrometric sized strips as an example of athree-dimensional structure and the method of making the same. Thecontactor includes metallic strips that are at a distance from eachother in the rest state and which are made by electrolytic means inseveral steps and secured to a substrate. The strips are formed of aniron and nickel alloy deposited by an electrolytic method. This alloyhas the property of being ferromagnetic so that the strips can be putinto contact with each other when a magnetic field passing through themcreates an attraction force between them. At least one aperture orconstriction is made on at least one of the strips to facilitate bendingof the strip.

The method of checking hermeticity has to be able to detect a leakagerate from very small closed cavities of micrometric components whosevolume is less than 1 mm³, for example of the order of 0.02 mm³. Longequalization time constants of at least 20 years must be guaranteed forthe closed cavity of these micrometric components in order for thestructure of each component to be protected from any contaminatingfluid. The contaminating fluid can be a liquid or a gas.

When the cap of the micrometric component is fixed onto the substrate toenclose the structure to be protected, micro-cracks may be observed,liable to allow an external contaminating liquid or gas to penetrate thecavity and contaminate said structure.

In order to check hermeticity or detect leakage from a closed cavity ofa component via a conventional method, a checking gas has to beintroduced into the cavity. In order to do this, said component has tobe placed in a container or an enclosure which is filled with a gas,such as helium at a high pressure in order to accelerate theintroduction of the gas into said cavity. However, one drawback of thismethod is that the measuring gas introduced into the cavity is liable toescape partially or entirely if there is a large leak, which can distortthe hermeticity check which is conventionally carried out using a massspectrometer.

Another drawback of the conventional method is that the leakagedetection threshold of a mass spectrometer system is of the order of5·10⁻¹² mbar·l/s. This means that the maximum leakage rates of the orderof 10⁻¹⁵ to 10⁻¹⁴ mbar·l/s, which guarantee equalization constants of atleast 20 years for cavities with a volume of less than 1 mm³, cannot bedetected.

U.S. Pat. No. 6,223,586 discloses a method of inspecting leaks fromelectronic components, such as microelectromechanical devices having aclosed cavity. In order to do this, plates of such electronic componentsare placed first of all in a liquid bath under pressure for a determinedtime period. The liquid under pressure can be water for example. Oncethis step is finished, the components are placed in another liquid andan inspection can be carried out via microscope to detect areas ofleakage in the cavities of the components. This other liquid may also bewater so as to prevent the water inside the cavity evaporating. Anothermajor drawback of this method lies in the fact that the hermeticitychecking sensitivity is greatly reduced.

The main object of the invention is therefore to overcome the drawbacksof the prior art by providing a cumulative type method of checking thehermeticity of a closed cavity of at least one micrometric component,with remarkably increased sensitivity. This method has the capacity toreveal large leakages in a single detection operation. Means are thusprovided in each cavity in order to react with a quantity of fluid thathas penetrated the cavity during a determined period of time forchecking hermeticity.

SUMMARY OF THE INVENTION

The invention therefore concerns a method of measuring the hermeticityof a closed cavity of at least one micrometric component citedhereinbefore, which is characterized in that it includes steps of:

placing the micrometric component in a container, said componentcomprising inside a cavity a hermeticity check indicator element whoseoptical or electrical properties change permanently in the presence of areactive fluid capable of reacting with the indicator element;

hermetically sealing the container which includes said component;

filling the container with a reactive fluid under pressure in order tosubject said component to a higher fluid pressure than the pressure inthe cavity during a determined period of time, such as several days, thecontainer including means for introducing the reactive fluid, and

-   -   checking the variation in the properties of the indicator        element by optical or electrical means depending upon the        quantity of reactive fluid that has penetrated the cavity and        reacted with the indicator element to determine the hermeticity        of said cavity.

One advantage of the method of checking the hermeticity of the cavityaccording to the invention, lies in the fact that the indicator elementplaced in the cavity allows a certain quantity of reactive fluid thathas penetrated the closed cavity to be absorbed or to react.Consequently, the indicator element cumulates the effect of thisquantity of fluid that has penetrated the cavity and its optical orelectrical properties are thus permanently altered. Because of this, alarge leak or a small leak can be checked in the same way in a singleoperation using the hermeticity checking method.

Preferably, several wafers, which each include several micrometriccomponents made on the same substrate and having a plate of caps fixedonto the substrate to close each structure of the micrometriccomponents, can be placed in the container. In order to reduce thechecking time for a multitude of micrometric components, the wafers areall subjected to the reactive fluid pressure in the container for adetermined period of time. These wafers can be placed in the containerat the end of the manufacturing process of said wafers and before orafter electric test operations on each component. In each cavity of themicrometric components, an inert gas, such as argon, at a pressure closeto atmospheric pressure, can protect each structure.

Advantageously, the container filled with reactive fluid can be heatedto a temperature higher than the ambient temperature, preferably to atemperature higher than 100° C. by heating means during the determinedtime period. This accelerates the reaction of the reactive fluid thathas penetrated each cavity with the indicator element. This reactionacceleration can also be obtained using ultraviolet illumination (UV).

Advantageously, the container is filled with a reactive gas under apressure that is preferably greater than 10 bars, for example 15 bars soas to accelerate the introduction of the reactive gas in the cavities ofthe components. Since an equalization time of at least 20 years has tobe guaranteed for these micrometric components, the determined timeperiod for checking hermeticity can thus be reduced to several days witha pressure of 15 bars. This enables a sufficient quantity of gas to beintroduced into each cavity to react in a measurable way with theindicator element, even if the final concentration of the gas in thecavity remains less than 1%.

For a check via optical means, the indicator element of each cavity canbe a thin layer of copper or titanium, and the reactive gas is oxygen sothat the copper or titanium layer oxidises as a function of the quantityof oxygen that has penetrated the cavity. The thickness of the copper ortitanium layer, which is placed on the substrate or under the cap, canbe less than 100 nm, preferably equal to 30 nm. The copper or titaniumlayer changes colour and transparency when it oxidises at a determinedwavelength of at least one light beam emitted by a light source. If thesubstrate and/or the cap of each component are made of a transparentmaterial, the wavelength of the beam can be close to the infrared range,for example equal to 850 nm. If the substrate and/or the cap of eachcomponent are made of semi-conductor material such as silicon, thewavelength of the beam can be increased to 1.3 μm to pass therethroughwithout being absorbed.

For a check via electrical means, the indicator element of each cavityis for example a palladium resistor, and the reactive gas is hydrogen.The resistor is connected by conductive paths passing through themicrometric component for a resistance measurement from the exterior ofeach component.

The invention also concerns a micrometric component suitable forimplementing the aforementioned method, which is characterized in thatit includes an indicator element inside the cavity for checkinghermeticity, whose optical or electrical properties change permanentlyin the presence of a reactive fluid capable of reacting with theindicator element in order to check the hermeticity of the cavity ofsaid component.

Advantageously, the indicator element is a copper or titanium layer fora check via optical means for reacting with oxygen as the reactivefluid, or a palladium resistor for a check via electrical means forreacting with hydrogen as the reactive fluid. The cavity of eachcomponent is preferably filled with an inert gas, such as argon at apressure close to the atmospheric pressure.

Of course, the indicator element, particularly for an optical check, canbe chosen from among other metal materials, such as silver, zirconium orniobium.

For a check via optical means, the indicator element is formed of alayer of copper or titanium selectively etched or selectively depositedvia evaporation under vacuum through a mask on one part of the innersurface of the cap or on one part of the substrate area prior to theoperation of fixing the cap onto an area of the substrate. The thicknessof this layer of copper or titanium is comprised between 10 and 100 nm,preferably substantially equal to 30 nm. If the indicator element is ofcircular shape, the diameter of the copper or titanium layer iscomprised between 10 and 100 μm, preferably 70 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages and features of the method of measuring thehermeticity of a cavity of a micrometric component, and of saidmicrometric component, will appear more clearly in the followingdescription of embodiments of the invention with reference to thedrawings, in which:

FIG. 1 shows a three-dimensional partial cross-section of a micrometriccomponent showing a magnetostatic micro-contactor, according to a firstembodiment, in which an indicator element can be checked via opticalmeans for the hermeticity check,

FIG. 2 shows a three-dimensional partial cross-section of a micrometriccomponent showing a magnetostatic micro-contactor, according to a secondembodiment, in which an indicator element can be measured via opticalmeans for the hermeticity check,

FIG. 3 shows in a simplified manner a vertical cross-section of athermal container in which wafers of micrometric components are placedfor implementing the hermeticity checking method according to theinvention,

FIG. 4 shows in a simplified manner a three-dimensional view of ameasuring machine for checking, via optical means, the indicator elementof certain micrometric components of a wafer of components forimplementing the hermeticity checking method according to the invention,

FIG. 5 shows schematically a partial cross-section of one part of awafer of micrometric components of the first embodiment of FIG. 1, andchecking means of the measuring machine for implementing the hermeticitychecking method according to the invention,

FIG. 6 shows schematically a partial cross-section of one part of awafer of micrometric components of the second embodiment of FIG. 2, andchecking means of the measuring machine for implementing the hermeticitychecking method according to the invention,

FIG. 7 shows a three-dimensional partial cross-section of a micrometriccomponent with a micro-contactor, according to a third embodiment, inwhich an indicator element can be checked via electrical means forchecking hermeticity,

FIG. 8 shows a graph of the partial oxygen pressure as a function ofexposure time in a closed cavity of a micrometric component for a methodof checking hermeticity via optical means according to the invention,and

FIG. 9 shows graphs of the optical transmission of a layer of oxidisedcopper, of a non-oxidised layer and of the contrast ratio between layersof oxidised and non-oxidised copper as a function of the wavelength of alight beam passing through said layer for the method of checking viaoptical means according to the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

In the following description, the means for implementing the hermeticitychecking method for a closed cavity of at least one micrometriccomponent, which are well known to those skilled in the art, are shownand explained in a simplified manner.

FIG. 1 shows a first preferred embodiment of a micrometric component 1for implementing the hermeticity checking method via optical means. Saidmicrometric component 1, shown in FIG. 1, comes from a wafer of severalmicrometric components, which are made on the same semi-conductorsubstrate 2, such as silicon or glass, after the operation of dicingsaid wafer. However, for the hermeticity checking method for a closedcavity 14 of at least one micrometric component 1, it may be a separatecomponent from the wafer or at least one component of the wafer prior toa dicing operation. The wafer, which is not shown in FIG. 1, can includethousands of micrometric components.

Micrometric component 1 includes a three-dimensional structure 5, 6 and10 made over one portion of a substrate 2, a cap 3 for protecting thestructure fixed onto a zone of substrate 2 and an indicator element 4sensitive to a specific reactive fluid. The indicator element 4 isplaced on a top part of the inner surface of cap 3 in this firstembodiment. A closed cavity 14 is delimited by the inner surface of cap3, the three-dimensional structure 5, 6 and 10 and the zone of substrate2.

The volume of one cavity 14 of such a micrometric component 1 is of theorder of 0.02 mm³ (1000 μm long, 200 μm wide and 100 μm high). Thiscavity 14 with a very small volume is preferably filled with an inertgas, such as argon, at a pressure close to atmospheric pressure.

The three-dimensional structure 5, 6 and 10 can be a magneticmicro-contactor. For technical details regarding the making of thismicro-contactor, the reader can refer to EP Patent No. 0 874 379 by thesame Applicant which is cited herein by way of reference.

This micro-contactor is formed of a first conductive strip 6, one end ofwhich is secured to substrate 2 via a conductive foot 5, and the otherend of the first strip is free to move, and a second conductive strip 10fixed onto the substrate. Each metal strip 6 and 10, and conductive foot5 can be obtained by an electrolytic method. In the presence of amagnetic field, the free end of first strip 6 can come into contact withthe second strip 10.

A first conductive path 8 connects conductive foot 5 of the first strip6 to a first electric contact terminal 9 placed outside cavity 14 onsubstrate 2. A second conductive path 11 connects second strip 10 to asecond electric contact terminal 12 placed outside cavity 14 onsubstrate 2. Each contact terminal 9 and 12 can be connected to thecontact terminals of an electronic circuit or to contact terminals of anelectronic apparatus. An integrated circuit that is not shown can bemade underneath the micro-contactor and electrically connected to eachmetal strip 6 and 10 of said micro-contactor. Of course, the electricconnection of the micro-contactor to the exterior of cavity 14 can alsobe achieved via metallized holes through substrate 2 or any other mannerdifferent to that shown in FIG. 1.

An intermediate part of first strip 6 of the micro-contactor has anaperture 7 extending over most of its length to facilitate the bendingof strip 6. The distance separating foot 5 of first strip 6 and one endof second strip 10 approximately corresponds to the length of aperture7.

The indicator element 4, used for checking the hermeticity of closedcavity 14 of micrometric component 1, is preferably a copper layer, butcan also be a titanium layer, which is placed at a distance from andopposite aperture 7. This copper or titanium layer can be obtained byselective chemical etching or by selective vapour deposition over a toppart of the inner surface of the cap before cap 3 is secured tosubstrate 2. The chemical etching technique may be difficult toimplement to make the copper or titanium layer in a cavity. It is thuspreferable to use technique of deposition by evaporation under vacuum ofcopper or titanium in this first embodiment. Preferably, a copper layeris deposited which is then easy to achieve with this deposition method.

Of course, deposition by evaporation under vacuum of the copper layer orthe titanium layer can be carried out during the steps of manufacturingthe wafers of micrometric components. In order to do this, caps arefirst of all made for example by chemical etching in a glass or siliconplate. After this step, the copper or titanium layer is selectivelydeposited at the same time on the inner surface of caps 3 of at leastone plate of caps through a mask in which holes are made ofsubstantially equal dimensions to the dimension of the copper layer tobe deposited. The mask used can be for example a photo-structurableglass mask of the Foturan type from the Schott Glas company in Germany.

Once the copper layers have been selectively deposited on each innersurface of caps 3 of the plate of caps, the plate is fixed ontosubstrate 2 of the wafer. Thus, each cap 3 of the plate is fixed ontoeach corresponding zone of substrate 2 in order to enclose a respectivemicro-contactor of the wafer to be protected. This plate of caps can bemade for example of glass or silicon.

The thickness of this copper layer is comprised between 10 and 100 nm,preferably substantially equal to 30 nm to allow a light beam ofdetermined wavelength to pass through said layer for a hermeticity checkvia optical means. It should be noted that the smaller the thickness ofthe copper layer, the greater the sensitivity, but with decreasedcontrast. A compromise thus had to be found between contrast andsensitivity, which means that the thickness of the copper layer waspreferably chosen to be equal to 30 nm.

The surface dimension of the copper layer can be of smaller dimensionthan the surface dimension of aperture 7 of the micro-contactor. Thisallows first and second measuring zones to be defined through aperture7, as explained hereinafter with reference particularly to FIGS. 5 and6. If said layer has a circular surface, the diameter of the layer canbe approximately 70 μm so that it can be easily measured via opticalmeans using conventional means. Thus, each copper layer 4 deposited onthe inner surface of each cap 3 has a mass of approximately 1.03 ng or1.63·10⁻¹¹ mole.

Cap 3 or each cap of the plate of caps is fixed onto each zone ofsubstrate 2 via an annular glass frit sealing gasket or preferably ametal alloy ring 13. The metal alloy can be an alloy composed of goldand tin (Au—Sn) with a melting point of the order of 280° C. Of course,the metal alloy chosen must provide proper adherence to the materialschosen for substrate 2 and cap 3. Before securing the cap via this metalalloy, a conventional insulating layer has to be provided, not shown, onconductive paths 8 and 11.

FIG. 2 shows a second preferred embodiment of a micrometric component 1for implementing the hermeticity checking method via optical means. Itshould be noted that those elements of FIG. 2, which are the same asthose of FIG. 1, bear identical reference signs. Consequently for thesake of simplification, the description of these elements will not berepeated.

The essential difference of this second embodiment as regards the firstembodiment of the micrometric component is the fact that the copper ortitanium layer 4 is made directly on substrate 2 for example before themicro-contactor is made. This copper or titanium layer 4 is positionedopposite aperture 7. The surface dimension of the layer is less than thesurface dimension of the aperture. This enables first and secondmeasuring zones to be defined through the aperture for a check viaoptical means in the vertical direction of the component.

It is possible to make the copper or titanium layer 4 by selectivechemical etching or deposition techniques by evaporation under vacuum asexplained hereinbefore. It should be noted that chemical etching in thissecond embodiment can also be used given that the copper layer is madeon a flat surface of substrate 2, for example on an insulating layer.

Since the copper or titanium layers of this second embodiment are madeduring manufacturing of the wafers on the substrates, the manufacturingtime of said wafers may be slower than the manufacturing time of thewafers of the first embodiment of the micrometric components. However,the dimensions of this copper or titanium layer for each cap 3 may beequal to those mentioned in the first embodiment.

With reference to FIG. 3, a thermal container 30 is shown in a verysimplified manner for the explanation of the first steps of the methodof checking the hermeticity of several micrometric components. Ofcourse, all of the elements of the container can take different formsfrom those shown in FIG. 3, provided that the first steps of the methodcan be carried out as normal.

Thermal container 30 essentially includes a tub 31 and a cover 32 hingedabout an axis of rotation 34 at the mouth of tub 31. Closing means 33are provided for hermetically sealing container 30 by applying cover 32to the top edge of tub 31. These closing means 33 are for example formedby a screw 33 a, whose head abuts against a top surface of cover 32, anda nut 33 b, abutting against the bottom surface of the top edge of thetub 31. The shank of screw 33 a passes through an aperture 33 c, whichis made in cover 32 and on the top edge of tub 31 in order to be screwedinto nut 33 b. The shape of the head of screw 33 a is suitable formanual handling. A sealing gasket, not shown, must also be provided atthe contact between the top edge of tub 31 and cover 32 in order to sealcontainer 30 hermetically.

Thermal container 30 also includes means 35 to 39 for introducing thereactive fluid, particularly oxygen inside tub 31, and heating means 40and 41 for heating the interior of tub 31. The heating means are mainlycomposed of a heating body 41 arranged inside the container and aheating control device 40 which controls the heating body to obtain adesired temperature in container 30.

The means for introducing oxygen under pressure into the tub are formedby a three-way control valve 35, and an apparatus 36 for measuring thepressure inside container 30. A first pipe 37 of the three-way controlvalve 35 is connected to an oxygen bottle that is not shown for filling,via a second pipe 38 of valve 35, the container with oxygen at apressure p when the first and second pipes are open and the third pipe39 is closed. At the end of the first steps of the checking method,container 30 is depressurised by withdrawing the oxygen under pressurefrom the container. In order to do this, the second and third pipes 38and 39 are open and the first pipe 37 is kept closed.

The first steps of the hermeticity checking method are explained moreprecisely hereinafter. First of all, several wafers 1′ of micrometriccomponents are placed in an open receptacle 43. This receptacle 43carrying wafers 1′ is then placed on a base 42 arranged on the bottom oftub 31 of container 30.

Container 30 is then hermetically sealed by closing means 33 by placingcover 32 abutting against the top edge of tub 31. Once container 30,which includes wafers 1′ of micrometric components, is hermeticallysealed, the first and second pipes 37 and 38 of valve 35 are openwhereas the third pipe 39 is closed. In this manner, container 30 can befilled with oxygen at a determined pressure, which can be controlled bymeasuring apparatus 36. Of course, the valve can be controlledelectronically so as to close all of pipes 37 to 39 when the oxygeninside the container is at the desired determined pressure.

The oxygen pressure in container 30 can be higher than 10 bars andpreferably substantially equal to 15 bars or 20 bars. Thus, all of themicrometric components are subjected to high oxygen pressure. In thismanner, the period of time necessary for introducing a detectablequantity of oxygen into each cavity and for guaranteeing an equalisationtime greater than 20 years can be greatly reduced as explainedhereinafter with reference to FIGS. 8 and 9.

In order to ensure that the oxygen introduced will react quickly withthe copper indicator element, the container is heated by the heatingmeans to a temperature higher than 100° C., for example 150° C. Thedetermined time period for causing the copper layer to oxidise as theindicator element of each cavity, if the oxygen has penetrated thecavity, is thus reduced to several hours, or several days (65 hours).Since several wafers of approximately 5,000 components each are placedin the container, it is possible to carry out these first steps of themethod for close to 500,000 micrometric components at the same time,which saves considerable time. Moreover, the optical measuring method onthe wafer prior to the dicing step allows hermeticity to be checkedquickly.

In order to better understand the time saving obtained by the firststeps of the method for ensuring the hermeticity of the closed cavity ofthe micrometric components, reference is made first of all to FIG. 8.FIG. 8 shows a graph of the partial oxygen pressure as a function ofexposure time in a closed cavity of a micrometric component.

When the micrometric component is placed in a normal open airenvironment, said component is in the presence of approximately 20%oxygen, which corresponds to an oxygen partial pressure of 200 mbar atthe equilibrium. Since the micrometric component cavity is filled onlywith an inert gas, such as argon, at a pressure of 800 mbar for example,there is thus a difference of 200 mbar of oxygen between the exteriorand interior of the cavity. Consequently, if there is a leak in thecavity, the oxygen will gradually re-enter said cavity in an exponentialasymptotic manner until there is an equilibrium of partial oxygenpressure.

As soon as the micrometric component is placed in the open air, thepartial oxygen pressure inside the cavity as a function of time can bedefined by the formula p=p₀·(1−e^((−t/T))). In this formula, p₀ is theoxygen pressure at the equilibrium, namely 200 mbar, and τ is theexchange time constant determined by the leakage rate. This exchangetime constant must be at least 20 years to ensure sufficient hermeticityfor the cavity in most cases of micrometric components. The slope of thecurve at the origin is thus defined by p₀/T. If the micrometriccomponents in the closed container are subjected to an oxygen pressureof for example 20 bars, the slope at the origin is greater and the speedof introduction of oxygen is speeded up by a factor of 100.

It should be noted that the copper layer used as an indicator element ineach cavity can oxidise in several hours in the presence of partialoxygen pressures greater than or equal to 1 mbar, i.e. approximately 100times lower than the pressure at the equilibrium. Consequently, when themicrometric components are subjected to an oxygen pressure of 15 bars or20 bars at a temperature of for example 120 or 150° C., the copper layertends to oxidise simultaneously with the introduction of oxygen insidethe cavity.

In each closed cavity of volume V close to 0.02 mm³, the copper mass isapproximately 1.03 ng with a copper layer with a diameter of 70 μm and athickness of 30 nm. With this type of copper layer, 4.08·10⁻¹² mole ofoxygen is sufficient to form a layer of copper oxide that can easily bedetected in transmission. In order to guarantee an equalization timeconstant of 20 years with a partial ambient pressure of 200 mbar, theleakage rate defined by the formula L=p₀·V/T is 6.34·10⁻¹⁵ mbar·l/s. Bymolecular flux conversion defined by the formula (dn/dt)=L/R·T where Ris 8.31 J/K·mole and T is 393 K, the oxygen leakage must be less than1.94·10⁻¹⁹ mole/s.

In a container at 20 bars oxygen, this leakage will be 100 timesgreater, i.e. 1.94·10⁻¹⁷ mole/s. Thus, the micrometric component wafersneed a little more than 58 hours of exposure in the container in orderfor the 4.08·10⁻¹² mole of oxygen necessary for quantifiable oxidisationof the copper layer to be able to be introduced. As describedhereinbefore, at least 1 mbar of oxygen pressure has to be reached inthe cavity, which means 4.9·10⁻¹³ mole, corresponding to an additionaldead time of 7 hours independently of the volume of the cavity. Thus,the micrometric component wafers must be left in the container under anoxygen pressure of 20 bars at a temperature of 120° C. for a determinedtime period of 65 hours.

After the determined time period, container 30 shown in FIG. 3 isdepressurised by opening the second and third pipes 38 and 39, andcooled. Then the container is opened to take out receptacle 43 carryingwafers 1′ for the optical checking operations in accordance with themethod of the invention. These optical checking operations are explainedin particular with reference to FIGS. 4 to 6 and 9.

FIG. 4 shows a measuring machine 50 for checking, via optical means, thehermeticity of the cavity of at least certain components 1 of wafers 1′.All of the elements of the machine, which are well known in thetechnical field of optical checking, are shown in a very simplifiedmanner. Of course, each element shown for optical measurement can be ofa different shape from that shown in FIG. 4.

Measuring machine 50 includes a base 54 on which there is mounted amoving support 51 which can be moved along the X and Y directions.Schematically shown means 56 are used as a guide for moving support 51on base 54. Moving support 51 includes a recess 53 at its centre givingit a U shape. One edge 52 on the side of recess 53 is made on the toppart of moving support 51 in order to be able to position and hold awafer 1′ of micrometric components 1 to be checked. Wafer 1′ is thuspositioned at a distance from the low part of recess 53.

For the optical measurement, measuring machine 50 includes a lightsource 20 fixed to the free end of an arm 57, the other end of which isfixed to a front wall 55 of measuring machine 50. The measuring machinealso includes an image sensor 21 fixed to the free end of an arm 58, theother end of which is fixed to the front wall of the machine. Electricalpowering and data processing means, not shown, are arranged in themeasuring machine.

Arm 57 carrying light source 20 at its free end is located in recess 53provided on moving support 51 between the low part of the support andthe back of wafer 1′ to be checked. After micrometric component wafer 1′has been positioned using moving support 51 to check the indicatorelement 4 of one of components 1, light source 20 is switched on. Thislight source 20 provides at least one beam of light IR of a determinedwavelength that passes perpendicularly through the wafer in one of themeasuring zones of the positioned micrometric component 1. The imagesensor positioned above wafer 1′ receives light beam IR in order todetermine the hermeticity of the component, as a function of thetransparency or colour of the copper layer.

The wavelength of the light beam from the light source must be close to850 nm in the near infrared if the substrate and each cap are made ofglass, or close to 1.3 μm if the substrate and/or each cap are made of asemiconductor material, such as silicon. In the first case, LED diodescan be used (880 nm or 950 nm) or a semiconductor laser (780 nm) aslight source 20 and a CCD type sensor as image sensor 21. In the secondcase, a 1.3 μm semiconductor laser is used and a suitable infraredimaging system as image sensor 21.

Preferably, as shown in the two embodiments of FIGS. 5 and 6 a firstlight beam IR1 is emitted by the light source passes through onemicrometric component 1 of wafer 1′ passing through the firstmeasurement zone of aperture 7 through copper layer 4 in order to bepicked up by image sensor 21. After this, after wafer 1′ has been moved,a second light beam IR2 emitted by the light source passes through saidcomponent passing through the second measurement zone of aperture 7 nextto the copper layer. In this manner, a comparative optical measurementcan be carried out. This comparative measurement can enable the dataprocessing means to calculate a relative optical transmission ratiobetween an oxidised copper layer and a non-oxidised copper layer. Thus,a hermeticity check or a leakage rate of the component can bedetermined.

FIG. 9 shows an optical transmission graph of the copper layers of 30 nmthickness in an oxidised state or in a non-oxidised state as a functionof the light beam wavelength. It is clear in FIG. 9 that the differencein transmission of a light beam through an oxidised copper layer and anon-oxidised copper layer increases with wavelength beyond 580 nm.

While the copper layer becomes increasingly opaque towards the infrared,the copper oxide becomes increasingly transparent, hence a highmeasurement contrast. By determining the transmission ratio Tox/Tcubetween the oxidised copper layer and the non-oxidised copper layer, oneobtains a reference defined at 100% at 850 nm wavelength of the lightbeam provided by the light source and picked up by the image sensor. Itshould be noted that the transmission ratio in the infrared is higherthan 10, while in the green, this factor is less than 2.5.

FIG. 7 shows a third embodiment of a micrometric component 1 forimplementing the method for checking hermeticity via electric means. Itshould be noted that those elements of FIG. 7, which are the same asthose of FIGS. 1 and 2 bear identical reference signs. Consequently, forthe sake of simplification, the description of these elements will notbe repeated.

For checking the hermeticity of cavity 14 of micrometric component 1 viaelectric means, instead of the copper layer, there is provided apalladium resistor 15 housed entirely in said closed cavity. In order tohave a significant resistor value capable of being easily measured by ameasuring apparatus, the resistor describes a coil in the cavity. Ofcourse, the length of the coil can be much greater than that illustratedin FIG. 7. This palladium resistor can be made by an electrolyticprocess, by chemical etching or any other known manner.

A third conductive path 16 connects a first end of resistor 15 to athird electric contact terminal 17 placed outside cavity 14 on substrate2. A fourth conductive path 18 connects a second end of resistor 15 to afourth contact terminal 19 placed outside cavity 14 on substrate 2. Eachcontact terminal 17 and 19 can be connected to contact terminals of anelectronic circuit or to contact terminals of an electronic measuringapparatus, which are not shown.

The value of the palladium resistor has the ability to change in thepresence of hydrogen as the reactive fluid. It should be noted thatpalladium has the property of absorbing at ambient temperature up to 900times its volume of hydrogen. This corresponds to an atomic fraction ofapproximately 50%. Consequently, because of this palladium resistor 15in cavity 14 of micrometric component 1 and the checking methoddescribed hereinbefore, it is possible to check the hermeticity of saidclosed cavity via electric means or to define a rate of leakage fromsaid cavity.

Owing to the indicator elements placed in each cavity of the micrometriccomponents of one or several wafers of components, it is possible tocarry out indirect measurements via the accumulation in the indicatorelement of the reactive fluid that has penetrated the cavity. Theindicator element in the cavity changes its optical or electricproperties as a function of the reactive gas introduced into the cavity.This provides the advantage of enabling the same hermeticity checkingmethod to be used for micrometric components have large or fine leaksfrom their cavity.

From the description that has just been given, multiple variants of themethod for checking the hermeticity of a closed cavity of at least onemicrometric component can be devised by those skilled in the art withoutdeparting from the scope of the invention defined by the claims. In thecase of a check via optical means, a measurement of the copper layer canbe carried out by the reflection of a light beam on the copper layerinstead of transmission through the micrometric component. This copperlayer can completely cover the inner surface of each cap and/or eachinner zone of the substrate. Optical checking of the copper layer can beconducted using measuring means arranged in the container, or visuallyunder a microscope for example.

1. A micrometric component including: (a) a structure made on or in oneportion of a substrate; and (b) a cap fixed to one zone of the substrateto protect the structure, wherein a closed cavity is delimited by theinner surface of the cap, the structure and the zone of the substrate,wherein the micrometric component further includes, inside the closedcavity, an indicator element formed of a copper layer having a thicknessin the range of 10 to 100 nm which can check for hermeticity of theclosed cavity, and wherein optical properties of the copper layer changepermanently in the presence of a reactive fluid capable of reacting withthe copper layer, and by measuring the changes of the optical propertiesof the copper layer, the copper layer can check hermeticity of theclosed cavity, wherein the structure is a magnetic microcontactor whichincludes a first conductive strip, one end of which is secured to thesubstrate by a conductive foot, and the other end of the first strip isfree to move to come into contact with a second conductive strip fixedto the substrate in the presence of a magnetic field, an intermediatepart of the first strip having an aperture extending over most of itslength, the distance separating the foot of the first strip and one endof the second strip corresponding to the length of the aperture, andwherein the copper layer, which is at a distance from and opposite theaperture, has a surface dimension less than the surface dimension of theaperture to define, through the aperture, first and second measuringzones for the passage of light beams for checking hermeticity.
 2. Themicrometric component according to claim 1, wherein the closed cavityincludes an inert gas at a pressure close to the atmospheric pressure,wherein the reactive fluid is oxygen, and wherein the copper layer isobtained by selective chemical etching or by selective deposition byevaporation under vacuum over one part of the inner surface of the capor over one part of the zone of the substrate.
 3. The micrometriccomponent according to claim 2, wherein the copper layer has a thicknessclose to 30 nm.
 4. The micrometric component according to claim 1,wherein the substrate and/or the cap are made of glass or silicon.